Usually the stuff freestyle sales works as promised. A pinhole is no more than a box with a hole on one end and a film holder of some kind on the other.

A film back to hold the holder can be made from a 2'x2' piece of masonite... 2 bucks and a bit of window sash moulding...with enough left over to make two more.

The space between your lens board with a hole in it and the back can be anything. A big enough Alum can would work just fine. You could probably buy a cookie tin or coffee can to fill in the space. Then just add your pinhole.

I probably should make a crude 4x5 pin hole. I don't really care much for pinhole these days but everybody needs one lying about. I have a med format roll film on. I have the bones for a 2x3 cutfilm one laying about as well.